1. Field of the Invention
The present invention relates to a photovoltaic device and, more particularly, to a photovoltaic device which, by virtue of light confinement effect, exhibits improved photoelectric conversion efficiency, as well as improved weather resistance and durability, and which offers a greater yield in the production process.
2. Description of the Related Art
Various materials of photovoltaic device devices have been known, such as tetrahedral amorphous semiconductors and polycrystalline semiconductors, e.g., amorphous silicon, amorphous silicon germanium and amorphous silicon carbide, II-VI group compound semiconductors, e.g., CdS and Cu.sub.2 S, and III-V group compound semiconductors. e.g., GaAs, GaAlAs and so forth. Among these known materials, thin-film photovoltaic device devices having photoelectromotive force generating layer made of an amorphous semiconductor or a polycrystalline semiconductor are considered as being promising, because the use of such materials enables formation of a film of greater area than that made of a single-crystal silicon and reduces the required thickness of the film, while permitting deposition on any desired suitable substrate material.
Such a thin-film photovoltaic device, however, cannot provide photoelectric conversion efficiency of a high level which would compare with that of photovoltaic device using single-crystal silicon. It is necessary that improvements are achieved both in photoelectric conversion efficiency and durability, in order that this type of photovoltaic device is put to practical use.
Various attempts have been made with an aim to improve photoelectric conversion efficiency of thin-film photovoltaic devices. One of the requisites for achieving higher photoelectric conversion efficiency is to enhance absorption of light in the semiconductor layer of the thin film, so as to improve short-circuit current (Jsc). Enhancement of the light absorption is necessary because the thinner semiconductor layer which on the one hand meets the requirement for lower production cost and on the other hand reduces the light absorption as compared with devices using bulk semiconductors. There are five types of technology available for enhancing light absorption in thin-film semiconductor layer, as shown in (1) to (5) below.
(1) A technique has been known in which a reflective film having a high level of reflectance, such as of Ag, Al, Cu, Au or the like, is formed on the side of the photovoltaic device element opposite to the light incident side. According to this technique, the light which has passed through the carrier-generating semiconductor layer is reflected by the reflective film so as to be further absorbed by the semiconductor layer, so that the light absorption in the thin-film semiconductor layer is enhanced to provide a greater output current, whereby photoelectric conversion efficiency is improved.
(2) A method has been disclosed in Japanese {Patent Publication Nos. 59-43101 (Applicant: Fuji Electric Co., Ltd.)) and 60-41878 (Applicant: Sharp Corp.) in which the surface nature of a substrate is improved by a transparent conductive layer intervening between a back-side electrode and a semiconductor layer. According to these disclosures, the transparent conductive layer intervening between the back-side electrode and the semiconductor layer improves the smoothness of the back-side electrode, as well as adhesion of the semiconductor layer, and effectively prevents an alloying reaction between the metal constituting the back-side electrode and the semiconductor layer.
(3) Japanese Patent Laid-Open No. 60-84888 (Applicant: Energy Conversion Devices) discloses a technique in which a transparent conductive layer serving as a barrier layer is formed between a back-side electrode and a semiconductor layer, so as to reduce electrical current flowing through the defective region of the semiconductor layer.
(4) Y. Hamakawa et. al., Appl. Phys. Lett., 43 (1983), p644, reports that a transparent conductive layer of TiO.sub.2 interposed between an Ag back-side electrode and an amorphous silicon semiconductor layer serves to enhance sensitivity in longer-wavelength region in the spectral sensitivity of a solar cell.
(5) A technique has been proposed in which the surface of a back-side electrode is textured on the order of the wavelength of the light. According to this technique, long-wavelength light rays which have not been absorbed in the semiconductor layer are scattered so as to provide an effect that the light paths in the semiconductor layer are extended to improve the sensitivity of the photovoltaic device at the long-wavelength region. Consequently, short-circuit current is enhanced to improve photoelectric conversion efficiency. This technique is disclosed in T. Tiedje et. al., Proc. 16th IEEE Photovoltaic Specialist Conf., (1982), p 1423 and also in H. Deckman et. al., Proc. 16th IEEE Photovoltaic Specialist Conf., (1982), p 1425.
With the knowledge concerning these techniques, a photovoltaic device is recommended that has a back-side reflection layer made of a metallic film having a high level of reflectance and which has a light scattering texture of a size on the order of light wavelengths, while serving also as a back-side electrode, and that has a transparent conductive film provided between the back-side reflective film and the semiconductor layer.
It has been found, however, that practical production of a photovoltaic device having such a back-side electrode encounters the following problems (a) to (d) in regard to workability of the materials and durability of the products.
(a) It has been considered that the texture preferably has pyramidal shapes such as those disclosed in T. Tiedje, et. al., Proc. 16th IEEE Photovoltaic Specialist Conf. (1982), p1423, as such shapes are believed to provide superior light confinement effect. It has been discovered, however, that formation of an electrode and a semiconductor layer on the substrate having such a texture tends to cause a leak current through the defect region of the semiconductor layer to increase, thus impairing the yield of production of photovoltaic devices. In addition, the semiconductor layer formed on the substrate surface having such pyramidal texture has an effective thickness smaller than that of a semiconductor layer formed on a smooth substrate surface. Consequently, the effective thicknesses of layers such as a doping layer in the thin-film photovoltaic device, which are intentionally designed to have smaller thicknesses than those in existing devices, are further reduced, with the result that the open circuit voltage (Voc) and fill factor (FF) are reduced as compared with those of photovoltaic devices formed on a smooth substrate surface.
(b) The use of Ag or Cu as the material of the metallic back-side reflection layer involves a problem in that migration of Ag or Cu tends to occur, particularly under a positive bias voltage applied to the metallic back-side reflection layer in a highly humid atmosphere. Such migration allows electrodes on the light-incident side to conduct, thus causing a shunting of the photovoltaic device. It has been confirmed that this phenomenon is remarkable particularly when the metallic back-side reflection layer has a texture on the order of light wavelengths.
(c) Migration such as that observed with the use of Ag or Cu does not occur when Al is used as the material of the back-side reflection layer. In this case, however, the reflectance is lowered when the substrate has a texture. In some cases, reflectance was seriously lowered when a transparent conductive layer was superposed on the textured surface of the Al reflection layer.
(d) Use of a substrate and a back-side reflection layer having smooth surfaces rather than textured surfaces causes insufficient light absorption because the scattering of the light rays at the back side is not appreciable. In addition, such smooth surfaces tend to cause insufficient adhesion between the substrate and the back-side reflection layer, undesirably allowing separation of the back-side reflection layer from the substrate in the course of production of the photovoltaic devices.
These problems are serious particularly when the production process employs, for the purpose of low-cost commercial production, inexpensive substrate material such as a resin film, stainless steel or the like, and higher speed of formation of the semiconductor layer, with the result that the yield of the products is undesirably limited.